DescriptionBody with a central disc of up to 20 mm in diameter and 5 twisted arms of about four times that length; colour variable but often red and purple, the arms are often dark banded.

DiscThe dorsal side of the disc is covered with granules, often with small spines among them, covering the radial shields, but leaving the primary plates naked. The primary plates often form a conspicuous 10-radiate star. Radial shields as a rule completely concealed. On the ventral side, the granules and spines may be wholly absent (O. aculeata disc).

ArmsThe dorsalarm plates are oval and surrounded by a ring of small plates. The ventral arms plates are rectangular and often separated by a small depression. There are 6-7 short, thick, outstanding arm spines per segment. In the proximal part, the tube foot pores are partially covered by one (exceptionally two), rather large tentacle scale (O. aculeata arm dorsal, O. aculeata arm ventral).

MouthThe mouth shield is more or less oval, but rather variable in shape. There is one small infradental papillae on top of the jaw and there are 3 (rarely 2) mouth papillae on each side of the jaw. The vertical row of teeth is wedge-shaped and truncated (O. aculeata mouth).

HabitatIt lives on hard substrata where it nestles itself in hollows and crevices of stones, shells, etc.; rare below 300 m although it has been recorded of a depth of 1880 m.

DistributionIn the North Sea the species is found on the British east coast and on the Scandinavian coasts down to the Sound and the Belts. It is circumpolar distributed in the Arctic seas. Its southern limit in European seas is the British south coast. On the American side of the Atlantic it goes as far south as Cape Cod. In the Northern Pacific it goes as far south as California and Japan.